Control system of alternating current motors

ABSTRACT

In a control system of an alternating current motor energized by a frequency converter including serially connected rectifier and inverter of the type wherein the speed of the motor is controlled by varying the phase of the output current of the inverter by varying the phase of a control signal of the inverter, there are provided a function generator which is supplied with a signal related to the motor torque for generating an analog phase angle signal related to the current phase angle, and a phase shifter responsive to the analog phase angle signal for shifting the phase of the control signal for the inverter.

BACKGROUND OF THE INVENTION

This invention relates to a conrol system of an alternating currentmotor, and more particularly to a system of controlling the speed of anAC motor by using a frequency converter including serially connectedrectifier and inverter.

As shown in FIG. 1, an induction motor 15 is energized by an AC powersource 11 through a rectifier 12, a DC reactor 13 and an inverter 14.The rectifier and inverter comprise semiconductor switching elementssuch as thyristors or power transistors.

One example of the prior art control system is constructed as shown inFIG. 1 in which the outputs of a speed reference setter in the form of avariable resistor 21 and of a speed detector in the form of a tachometergenerator 16 coupled to an induction motor 15 are compared by acomparator 100. In response to the output of the comparator 100 a speedcontroller 22 applies a speed control signal to an absolute valueconvertor 23 and a function generator 27, the former forming a currentinstruction signal and the latter a slip frequency instruction signal.

The current instruction signal is compared with a current signaldetected by a current transformer 101 by means of a comparator 102, andthe result of comparison is applied to a current controller 24 to form acurrent control signal which is applied to a phase controller 25 forcontrolling the ignition of the thyristors of the rectifier.

The slip frequency instruction signal is compared with the output of thespeed detector 16 by means of a comparator 103 and the result ofcomparison is applied to an oscillator 29 to form a pulse signal. Thefrequency of this pulse signal is divided by a frequency divider 30 andthe output thereof is applied to a pulse amplifier 32 for controllingthe ignition of the thyristors of the inverter 14.

With the control system described above, the slip frequency and thesecondary current of the induction motor 15 are controlled in apredetermined correlated manner so as to maintain the magnetic flux ofthe motor at a substantially constant valve.

FIGS. 2a and 2b are vector diagrams showing the powering andregeneration mode operations of the motor, in which Φ represents themagnetic flux, E₁ the primary voltage, I₁ the primary current, E₂ thesecondary induced voltage, I₂ the secondary current, I₀ the excitingcurrent, φ the power factor angle, and θ the angle between the magneticflux Φ and the primary current I₁ and termed a current phase angle.

When the speed of an alternating current motor is controlled by acurrent control type frequency converter described above, the currentphase of the primary current I₁ is determined by the gate signal appliedto the thyristors as is well known in the art. Since the current phaseis fixed, under a transient state in which the operation of the motor ischanged from the powering mode to the regeneration mode or vice versa,the phases of E₁, E₂ and Φ vary to establish the desired phaserelationship for the new operation state.

The speed at which the phases of the induced voltage and the magneticflux vary is determined by the secondary time constant

    T.sub.2 =(L.sub.2 +L.sub.m)/R.sub.2

where

L₂ : secondary circuit inductance as viewed from primary side

L_(m) : excitation mutual inductance of the motor

R₂ : secondary circuit resistance as viewed from primary side.

Generally, the secondary time constant T₂ is of the order of severalhundred milliseconds so that even when the polarity of the output of thespeed controller is reversed and an instruction for changing theoperation mode from powering to regeneration or vice versa is produced,desired torque would not be produced until the phase of flux Φstabilizes at a desired phase angle. For this reason, under thesetransient states, even when the primary current is increased the torquewould not increase correspondingly. Furthermore, when the load variesduring the powering or regeneration mode operation the phase of the fluxΦ should be varied in order that the power factor angle φ becomes apredetermined value so that the torque would not increase correspondingto the increase in the current under transient state. In short, with theprior art control system, since the current phase is fixed it isimpossible to obtain desired torque under transient state and hencequick response speed control.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved control systemof an alternating current motor which can quickly vary the torque inresponse to load variation.

Another object of this invention is to provide a control system of analternating current motor energized by a rectifier-inverter typefrequency converter wherein the torque of the motor can be variedcommensurate with the variation of the motor current independently ofthe secondary time constant of the motor thus simplifying the controlsystem.

Still another object of this invention is to provide a control system ofan alternating current motor capable of varying the torque of the motorat high response speed when the load of the motor changes thusefficiently utilizing the motor capacity.

According to the invention, there is provided a control system of analternating current motor energized by a frequency converter of theclass wherein the speed of the motor is controlled by varying the phaseof the output current of the frequency converter by shifting the phaseof a control signal of the frequency converter, wherein there isprovided a function generator supplied with a first signal related to atorque generated by the motor for generating a second signalcorresponding to the phase of a primary current of the motor, and aphase shifter for shifting the phase of said control signal inaccordance with the second signal.

According to a preferred embodiment of this invention there is provideda control system of an alternating current motor energized by afrequency converter including serially connected rectifier and inverterof the class comprising a speed controller which generates a speedcontrol signal in response to the speed of the motor and a referencesignal, a current controller for generating a current control signal inresponse to the speed control signal and current flowing through thefrequency controller, a phase controller responsive to the currentcontrol signal for controlling the phase of a first control signal forthe rectifier, means responsive to the speed control signal forgenerating a slip frequency signal, an oscillator responsive to thespeed of the motor and the slip frequency signal, means connected to theoutput of the oscillator for generating a second control signal forcontrolling the inverter, characterized in that there are provided afunction generator responsive to the motor torque for generating ananalogue phase angle signal related to the phase angle between theprimary current and the magnetic flux of the motor, and a phase shifterconnected to receive the analogue phase angle signal, the output of theoscillator and the sum of the slip frequency signal and the speed signalfor generating the second control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing a prior art control system of analternating current motor energized by a frequency changer constitutedby serially connected rectifier and inverter;

FIGS. 2a and 2b show vector diagrams showing powering mode andregeneration mode operations respectively of the alternating currentmotor;

FIG. 3 is a block diagram showing one embodiment of this invention;

FIGS. 4a and 4b show equivalent circuits of an alternating currentmotor;

FIGS. 5a and 5b show the relationship between the magnetic flux and theprimary current during powering mode and regeneration mode and FIG. 5cis a graph showing the relationship between the current phase angle andthe primary current during the powering and regeneration modes;

FIG. 6a is a connection diagram showing a phase shifter utilized in thisinvention;

FIGS. 6b and 7 are waveforms for explaining the operation of the phaseshifter;

FIG. 8 is a block diagram showing a modified embodiment of thisinvention; and

FIG. 9 is a partial block diagram showing still further modification ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of this invention shown in FIG. 3 comprises afunction generator 33 which performs a predetermined mathematicaloperation in response to a slip frequency signal fs for producing ananalogue phase angle signal related to a current phase angle θ, which isapplied to a phase shifter 31 connected between a frequency divider 30and a pulse amplifier 32. The phase shifter 31 is further supplied witha signal e₁ corresponding to the sum of the slip frequency signal fs andan actual speed signal fm and the output of the frequency divider 30 toform a control signal which is applied to the pulse amplifier 32.Although in FIG. 3, the compounding of a control signal for one phase isshown, a plurality of control signals are formed in the same manner fora polyphase circuit. The other component parts are identical to thoseshown in FIG. 1.

FIGS. 4a and 4b are equivalent circuits of an induction motor useful toexplain the operation of the embodiment shown in FIG. 3. FIG. 4b is asimplified circuit obtainable by assuming that the secondary leakagereactance X₂, the exciting current I₀ and the magnetic flux Φ have thesame phase. The following analysis concerns FIG. 4b.

When the magnetic flux Φ is controlled to be constant by the controlsystem shown in FIG. 1 or 3 under an assumption that the excitingcurrent I₀ is constant, from FIG. 4b. ##EQU1## where K₁ and K₂ areproportionally constants. More particularly, when the ratio of theprimary voltage E₁ to the output frequency f₀ is controlled to beconstant, the magnetic flux Φ and the exciting current I₀ becomeconstant and the slip frequency f_(s) and the secondary current I₂become proportional with each other. For this reason, it is possible todetermine the secondary current I₂ where the slip frequency f₂ is knownand the ratio E₁ /f₀ of the motor can be determined as a constant. As aconsequence, the angle between the primary current I₁ and the magneticflux Φ, that is the current phase angle θ is shown by

    θ=tan.sup.-l (.sbsb.I.sub.2.sub./I.sbsb.0)

thus, where the ratio of the rated secondary current I_(2n) to theexciting current I₀ is given, the current phase angle can readily begiven, thus ##EQU2## where I₂ (pu) represents a magnifying factorregarding rated value. Consequently, when the slip frequency f_(s), thecurrent ratio k or the I₂ (pu) and k are given, the current phase angleθ can be determined as a function thereof.

FIG. 5c is a graph showing the relationship between I₂ (pu) and θ ofFIG. 5a and FIG. 5b show the angle θ between the magnetic flux Φ and theprimary current I₁ wherein θ during the powering is shown as positivewhile that during the regeneration as negative.

FIG. 6a shows the circuit diagram of the phase shifter 31. As shown itcomprises an inverter OA₁ which inverts the polarity of an inputanalogue signal e₁ that varies in proportion to the output frequency f₀of inverter 14. Accordingly, inputs to semiconductor switches S₁ and S₂have the same magnitude but opposite polarity. There is further providedan integrator OA₂ which integrates input signal e₁ for one half periodof the output frequency f₀ so as to produce an output voltage which isconstant irrespective of the variation in the output frequency f₀. Aninverter OA₃ and semiconductor switches S₃ and S₄ which are similar toinverter OA₁ and semiconductor switches S₁ and S₂ are connected to theoutput of integrator OA₂. As a signal e₂ for alternately ON-OFF controlsemiconductor switches S₁ and S₃, and S₂ and S₄ can be used the outputof frequency divider 30 since it produces a signal corresponding to theoutput frequency f₀ and this signal is applied to inverter 14 throughpulse amplifier 32 for controlling the output frequency f₀ thereof.

Supposing that signal e₁ is positive, when switch S₁ is ON, a negativeinput signal is applied to integrator OA₂ whereas when switch S₂ is ON apositive input signal is applied to integrator OA₂. Then the output ofthis integrator changes from positive to negative polarity in responseto the positive input signal, whereas from negative to positive inresponse to the negative input signal. As above described, when switchesS₁ and S₂ are alternately rendered ON for 1/2 period of the outputfrequency f₀, the output signal for integrator varies alternatelybetween positive and negative values E_(z1) and E_(z1) which areconstant irrespective of the output frequency. When this voltage isapplied to inverter OA₃ and when switches S₃ and S₄ are alternatelyrendered ON for 1/2 period of the output frequency f₀, a sawtooth waveoutput e₀ as shown in FIG. 6b can be obtained having a period of 1/2 ofthe output frequency. With this connection, when a signal proportionalto the output frequency f₀ is supplied to input terminal e₁ of the phaseshifter, a sawtooth wave as shown by FIG. 6b appears at the outputterminal e₀. This output signal repeats the same wave shape at each onehalf period of the output frequency and has a constant crest valueE_(z1) irrespective of the output frequency f₀.

FIG. 7 shows waveforms showing the manner of processing the output ofthe phase shifter shown in FIG. 6. More particularly, when the phaseshifter is constructed such that signals are produced at thecross-points between output signal e₀ and horizontal line A or B,signals R₁₁ and R₁₂ having different phases can be produced.

By combining these operations, it is possible to control the currentphase of the induction motor. Thus, in the embodiment shown in FIG. 3,the function generator 33 generates an analogue phase angle signalrelated to the current phase angle in accordance with the slip frequencyf_(s), while the phase shifter 31 generates a sawtooth wave e₀ shown inFIGS. 6 and 7. The control signal is compounded at the cross-pointsbetween the sawtooth wave e₀ and the analogue phase angle signal andapplied to the inverter 14 via pulse amplifier 32 to control the phaseof the output alternating current. Of course, control signals of thesame number as the number of phases of the output alternating currentare formed.

FIG. 8 shows a modified embodiment of this invention in which the outputof the speed controller 22 is applied to the function generator 33awhereby it generates a function of angle θ of primary current I₁. Thisfunction generator 33a corresponds to the combination of functiongenerators 27 and 33 shown in FIG. 3 and generates an analogue phaseangle signal.

Thus, it is possible to establish a function equation showing therelationship between the secondary current I₂ and the current phaseangle θ from the equivalent circuit shown in FIG. 4a, or to obtain thecurrent phase angle signal from the functional relationship between itand a signal related to the motor torque. Among signals related to themotor torque are included signals representing secondary current I₂,primary current I₁, a torque reference, a slip reference f_(s), magneticflux Φ, and induced electromotive forces E₁ and E₂.

The analogous phase angle signal can also be formed by correcting thecurrent phase angle θ by a delay angle caused by commutation necessaryfor operating thyristors utilized in the rectifier and the inverter ofthe frequency converter. As is well known in the art, the thyristor gatesignal and the output current phase of such converter are not at thesame phase due to commutation, so that it is necessary to precorrect thedelay caused by this fact.

FIG. 9 shows still another modification of this invention in which phaseshifter 31 is interposed between oscillator 29 and frequency divider 30so as to divide the frequency of a phase shifted signal for determiningthe current phase.

The sawtooth wave and the control pulses R₁₁ and R₁₂ can also be formedby any other well known circuits than those illustrated in FIGS. 6 and7. It will be clear that many other well known circuits can be used forshifting the phase of the input control signal to the phase shifter byan angle corresponding to the analogue phase angle signal. Furthermore,it is also possible to produce a digital signal from the functiongenerator 33 and to shift its phase by means of an adder of subtractor.

As above described, according to this invention, there is provided acontrol system of an alternating current motor energized by a frequencyconverter wherein a second signal corresponding to the primary currentphase of the motor is produced from a first signal related to the motortorque and the frequency converter is controlled in accordance with achange in the second signal caused by a change in the first signal sothat it is possible not only to provide an anticipation control of thecurrent phase but also to vary the torque commensurate with the currentvariation. Moreover, as the variation in the torque is independent ofthe secondary time constant of the motor it is possible to avoidcomplication of the control circuit. Further, as it is possible tostepwisely vary the torque the response speed of the control system isincreased. In this manner, the capacity of the motor is efficientlyutilized so that it is possible to decrease its capacity and overcurrentvalue.

I claim:
 1. In a control system of an alternating current motorenergized by a current control frequency converter of the class whereinthe speed of the motor is controlled by varying the phase of the outputcurrent of the frequency converter by shifting the phase of a controlsignal of said frequency converter, the improvement which comprises afunction generator supplied with a first signal including a speedcontrol signal form by a speed controller in response to an actual speedof said motor and a speed reference signal for generating a secondsignal corresponding to the phase of a primary current of said motor,and a phase shifter for shifting the phase of said control signal inaccordance with said second signal.
 2. In a current control system of analternating current motor energized by a frequency converter includingserially connected rectifier and inverter of the class comprising aspeed controller which generates a speed control signal in response tothe speed of said motor and a reference speed, a current controller forgenerating a current control signal in response to said speed controlsignal and current flowing through said frequency converter, a phasecontroller responsive to said current control signal for controlling thephase of a first control signal for said rectifier, means responsive tosaid speed control signal for generating a slip frequency signal, anoscillator responsive to the speed of said motor and said slip frequencysignal, means connected to the output of said oscillator for generatinga second control signal for controlling said inverter, the improvementwhich comprises a function generator responsive to the motor torque forgenerating an analogue phase angle signal related to the phase anglebetween the primary current and magnetic flux of said motor, and a phaseshifter connected to receive said analogue phase angle signal, theoutput of said oscillator and the sum of said slip frequency signal andsaid speed signal for generating said second control signal.
 3. Thecontrol system according to claim 2 wherein said function generator isconnected between the output of said means for generating said slipfrequency signal and said phase shifter.
 4. The control system accordingto claim 2 wherein said function generator is connected between theoutput of said speed controller and said phase shifter.
 5. The controlsystem according to claim 2 which further comprises a frequency dividerconnected between said oscillator and said phase shifter.
 6. The controlsystem according to claim 2 which further comprises a frequency dividerconnected on the output side of said phase shifter.